1. Field of the Invention
The present invention relates to a method of determining an optimum transfer bias for transferring a developed image formed on an image bearing member onto a transfer medium, such as an intermediate transfer member or a transferring material, in an image forming apparatus such as a copying machine or a printer adopting an electrophotographic system or an electrostatic recording system.
2. Related Background Art
As a conventional example of an image forming apparatus adopting the electrophotographic system, a full-color image forming apparatus using an intermediate transfer system is known in which electrostatic latent images are formed on one or multiple photosensitive drums that each serve as an image bearing member, developed images (toner images) in respective colors that are yellow, magenta, cyan, and black are formed by sequentially developing the latent images using toners in the colors that each serve as a developer, these toner images are transferred (primarily transferred) onto a drum-shaped or belt-shaped intermediate transfer member serving as a transfer medium so that the toner images are superimposed on each other, and the toner images on the intermediate transfer member are transferred (secondarily transferred) onto a transferring material by one operation, thereby obtaining a recorded image. As another conventional example, a monochrome image forming apparatus is known in which only a toner image in black is formed on a photosensitive drum and the toner image is directly transferred onto a transferring material.
In such an apparatus, in a transfer process where primary transfer from a photosensitive drum serving as an image bearing member is performed, a conductive transfer roller or the like is used as transfer means. The transfer roller is used while being abutted against the photosensitive drum and an intermediate transfer member and is given electric charges necessary to transfer a toner image not through discharging but through charge injection. Consequently, it is advantageous because the amount of ozone generated is small.
By the way, it is known that the resistance of the transfer roller tends to fluctuate due to the temperature/humidity or energization in the apparatus. In particular, when an ion conductive transfer roller made of a material where an ion conductive agent or a surface-active agent is dispersed is used, the resistance fluctuations described above easily occur.
On the other hand, if an electronic conductive transfer roller where a conductive filler, such as carbon or a metallic oxide, is dispersed as a conductive agent is used, the resistance fluctuations due to the temperature/humidity or energization are suppressed. However, when the surface of the transfer roller becomes soiled by toner as a result of long-term use or the thickness of a photosensitive surface layer opposing the transfer roller is reduced due to wear, the overall resistance of construction elements including the transfer roller and the photosensitive drum changes.
In either case, the resistance fluctuations occur, so that even if a constant voltage is applied to the transfer roller as a transfer bias, a current flowing to the transfer roller fluctuates, which results in a problem that optimum toner image transfer becomes impossible.
In order to prevent the occurrence of a transfer failure ascribable to the resistance fluctuations of the transfer roller, a method described in Japanese Patent Application Laid-Open No. 2001-125338 is used, for instance. With this method, a relation between a voltage applied to the transfer roller and a current flowing to a transfer part is measured as a pre-processing process and a transfer bias applied to the transfer roller is optimally controlled in accordance with a result of the measurement.
With this control method, a constant voltage obtained through constant voltage control is applied to the photosensitive drum from the transfer roller during pre-rotation before image formation (image creation), a current value at that time is detected, an optimum voltage V0 necessary to obtain an optimum current I0 is calculated from a relation between the voltage applied to the transfer roller and the current flowing to the transfer part, and the voltage V0 is applied as a transfer bias at the time of transfer during the image formation. As a result of these operations, even if the resistance of the transfer roller fluctuates, it becomes possible to cause the optimum current to flow to the transfer part at all times.
It should be noted here that in this specification, the term “pre-rotation” refers to a time slot, during which each image forming means operates, in a time period from the transmission of a print signal from the outside to the image forming apparatus to the arrival of the first sheet of the transferring material to a transfer position (transfer portion) of a developer image in an image forming process.
Meanwhile, as a method of developing an electrostatic latent image formed on the photosensitive drum using toner, various developing methods are known. In particular, with a magnetic brush developing method using a two-component developer containing toner and a magnetic carrier, a uniform image is obtained with relative stability, so that this method is applied to a color developing system. With this magnetic brush developing method, however, when the surface of the carrier becomes contaminated with a toner component, it becomes impossible to sufficiently charge the toner and the developing efficiency of the toner is lowered. Consequently, this developing method has a shortcoming unique to a two-component developer that periodical replacement of the carrier is required.
In order to overcome this shortcoming, a developing method that uses a one-component developer composed of a magnetic toner and not containing a carrier is used. With this developing method, the carrier degradation problem does not occur, so that developer replacement becomes unnecessary. Consequently, this method is particularly suited for development in black that is frequently performed. This one-component developing method uses no carrier as described above, so that in order to give electric charges to the toner, a method described in Japanese Patent Application Laid-Open No. S54-43038 is used, for instance. With this method, electric charges are given to the toner through triboelectrification between the toner and a developer bearing member that is provided for a developing device for performing a developing operation and feeds the toner to the photosensitive drum.
In addition, there is a case where a sufficient amount of electric charges is not given to the toner only through the triboelectrification between the toner and the developer bearing member and therefore sufficient transfer efficiency is not obtained. In view of this problem, a method is adopted with which charge before transfer is performed using a corona charger before a transfer process. As a result of the charge before transfer, electric charges having the same polarity as the toner are given to a toner image on the photosensitive drum after development and the toner electric charge amount is adjusted so that the transfer efficiency increases.
When the transfer bias control described above is performed in an image forming apparatus that performs such charge before transfer, however, there occurs a problem described below.
When the charge before transfer is performed, the toner electric charge amount increases and the potential of the photosensitive drum also changes. The photosensitive drum surface potential displaces to a polarity side that is the same as the toner charge polarity and the transfer bias has a polarity opposite to the polarity of the toner. Therefore, when the charge before transfer is performed, a potential difference between the photosensitive drum and the transfer roller in the transfer part increases. The transfer bias control before the image creation is performed under a state of charge OFF before transfer, while the image creation is performed under a state of charge ON before transfer.
Therefore, the potential difference described above during the image creation is larger than that before the image creation. Consequently, if a voltage determined through the transfer bias control is applied at the time of transfer, an excess current flows and a so-called “re-transfer phenomenon” occurs in which toner once transferred onto a transfer medium, such as an intermediate transfer member or a transferring material, is re-transferred back to the photosensitive drum, which leads to a problem that an image density is lowered.